Introduction In the substitution reactions, the leaving group from the substrate is replaced with the nucleophile. Because of the nucleophile it is called nucleophilic substitution. The lone pair of electrons, present on the nucleophile is used to create a new bond with the carbon atom, from which the leaving group was separated. There are two different mechanisms of nucleophilic substitution: SN1 and SN2. The difference between the two depends on how the reaction occurs. The SN1 reaction is unimolecular, meaning that the rate of the reaction depends on the concentration of the substrate only. The concentration of the nucleophile does not affect the rate of the reaction.
In SN1 reaction, the leaving group leaves first. Forming a carbocation, then nucleophile forms the bond with the positively charged carbon. Therefore the reaction consists of two steps. An SN2 reaction is bimolecular, meaning that the rate of the reaction depends on the concentration of both the substrate and the nucleophile. In such a reaction, the leaving group leaves at the same step when the nucleophile forms the bond with the carbon, therefore the reaction consists of one step only. In the experiment a series of SN2 reactions will be run and the results will be used to test the how different factors affect the reaction. The tested factors will be the steric hindrance, nucleophilicity of the amine, and the nature of the leaving group. Steric hindrance
is a very important factor that determines whether the reaction will go under an SN1 or SN2 mechanism. If the substrate if primary, then it is easy for the nucleophile to reach the substrate without another atoms being in the way. If the substrate is secondary, then the other factors have to be taken into consideration in order for the reaction to be SN2. Tertiary substrates do not go under the SN2 mechanism. In this experiment only primary and secondary substrates will be used which will guarantee the SN2 mechanism of the reaction. Nucleophilicity is a factor, which shows how easy it is for the nucleophile to attack the substrate.
All the nucleophiles have lone pairs of electron, but it depends on the stability of the nucleophile. If the nucleophile is stable on its own, then it will not readily attack the substrate. If it’s not stable and carries a negative charge, then it will attack the substrate and replace the leaving group in one step, which describes SN2 mechanism of the substitution reaction. The nature of the leaving group also can affect the reaction. If the leaving group is stable without being attached to the carbon, then it will easily be removed and replaced by the nucleophile. In the experiment, the bromide ion and the iodide ion are acting as the leaving groups, and the difference between the two will be determined, as they affect the reaction differently by having different electronegativities.
In the reaction of a tertiary amine and an alkyl halide, the halide ion acts as the leaving group and the tertiary amine acts as a nucleophile. Since the reaction follows SN2 mechanism, the leaving group will leave at the same time as the nucleophile replaces it and forms the new bond with the carbon. The reaction is single step and the rate of the reaction will depend on the nature of the reactants and the factors, which affect the rate. Since no carbocation is formed during the reaction, there should be no side products in the reaction. In this experiment, seven reactions will be run, six of which will be used to test the factors that affect the SN2 reaction, and seventh will be used to form a precipitate from an unknown amine using the same
mechanism. The melting point of the precipitant will then be taken to identify the unknown amine. Experimental method: Part 1: Nature of Nucleophile Begin by obtaining 3 test tubes labeled 1-3. In test tube 1, add 5ml of the solvent (acetone: diethyl ether: pentane) in a 5:4:1 ratio. Next, add 20 drops of triethylamine and 10 drops of iodomethane. Shake the mixture and then let sit and record any observations. In test tube 2, add 5mL of the solvent.
Next, add 20 drops of tripropylamine and 10 drops of iodomethane. Shake the test tube and record any observations. Lastly in test tube 3, begin by adding 5mL of the solvent. Next, add 20 drops of di- isoproplyamine and 10 drops of iodomethane. Shake the test tube and record any observations. Part 2: Nature of Halide and leaving group Obtain 3 additional test tubes and label them 4-6. In test tube 4, add 20 drops of triethylamine and 15 drops of iodoethane. Heat the test tube on a sand bath and record any observations. In test tube 5, add 20 drops of triethylamine and 15 drops of 1- Bromopropane. Heat the test tube on a sand bath and record any observations.
In test tube 6, add 20 drops of triethylamine and 15 drops of 2-bromopropane. Heat the test tube on a sand bath and record any observations. Part 3: Unknown Amine Obtain 1 clean test tube. Add 40 drops of the unknown into the testtube. Next, add 20 drops of iodomethane and 2mL of the solvent. Let the test tube stand for 10 -15 minutes. Put the test tube on the ice bath so that it does not heat up too much. Set up the vacuum filtration and begin the filtration on the mixture in the test tube. Wash the product with the solvent and then let dry. Once the product is obtained and dried, calculate its melting point and compare with the list of unknowns. Table of Chemicals Triethylamine Tripropylamine Ethyldiisopropylamine Chemical Structure
Chemical Properties Stable Extremely flammable Explosive when mixed with air Hygroscopic Stable Flammable Highly flammable Corrosive Physical Properties Molar Mass: 101.19 g/mol MP: -115 oC BP: 90 oC State: liquid Solubility: H2O Color: none Molar Mass: 143.27 g/mol MP: -94 oC BP: 156 oC State: liquid Solubility:H20 (slightly) Color: none Molar Mass: 129.24 g/mol MP: -46 oC BP: 127 oC State: liquid Color: none Solubility: H2O (miscible) lodomethane lodoethane 1-bromopropane Chemical Structure Chemical Properties Stable Incompatible with strong oxidizing agents Flammable Stable
Light sensitive Stable Highly Flammable Incompatible with strong oxidizing agents Physical Properties Molar Mass: 141.94 g/mol MP: -64 oC BP: 42 oC State: liquid Color: yellow Solubility: H2O Molar Mass: 155.97 g/mol MP: -108 oC BP: 69-73 oC State: liquid Color: yellow Solubility: H2O Molar Mass: 123 g/mol MP: -108 oC BP: 71 oC State: liquid Color: none Solubility: H2O 2-bromopropane Acetone Diethyl ether Chemical Structure Chemical Properties Stable Flammable Incompatible with strong oxidizing agents Stable Flammable Good solvent Stable Light sensitive
Physical Properties Molar Mass: 122.99g/mol MP: -89 oC BP: 59 oC State: liquid Solubility: H2O Color: none Molar Mass: 58.08 g/mol MP: -94 oC BP: 56 oC State: liquid Solubility:H20 Color: none Molar Mass: 74.12 g/mol MP:-116 oC BP: 34.6 oC State: liquid Color: none Solubility: H20 Results Part 1: Nature of Nucleophile Test Tube Time Observation 1 Instantly The mixture turned cloudy yet colorless. 2 Two – four Minutes Slowly begins to get cloudy to continues to until it looks thick. 3 n/a No reaction Part 2: Nature of Leaving Group and Halide Test Tube Time Observation 4 About 2 minutes Turned slightly yellow and cloudy. 56-8 minutes. Slightly yellow. 610+ minutes Pale yellow. Part 3: Unknown Amine Test Tube Observation Melting point Unknown White precipitate formed and the test tube began to get warm. 168°C
Discussion The results of the experiments are presented above. The first sets of the reactions were run between different tertiary amines and iodomethane. Since the substrate and the leaving groups for these reactions are the same, the effect of the difference in nucleophiles will be observed. The reaction between triethylamine and iodomethanel Reaction1) was the fastest one. The reaction between tripropylamine and iodomethanel Reaction2) was slower than Reaction 1, since the nucleophile used was more stable. The reaction between ethyldiisopropylamine and iodomethane (Reaction 3) gave no results.
This could happen because the nucleophile was too stable and was not able to replace the leaving group in the substrate. The second set of the reactions was between triethylamine and various substrates. Since triethylamine gave the fastest reaction with iodomethane in the set one, it is expected to be a strong enough nucleophile for a SN2 reaction. The reaction between triethylamine and iodoethane (Reaction 4) was the fastest in the second set. However, Reaction 4 is slower than Reaction 1, because the leaving group in the Reaction 4 is attached to primary carbon, while the leaving group in Reaction 1 is attached to methyl carbon.
The reaction between 1-bromopropane and triethylamine (Reaction 5) went slower than Reaction 4. The reason for that is the nature of the leaving group. Iodide ion is more electronegative and therefore it is easier replaced by the nucleophile than bromide ion. The reaction between 2-bromopropane and triethylamine (Reaction 6) was the slowest reaction in the set, because a the leaving group was attached to a secondary carbon, which made it harder for the nucleophile to reach for that carbon. The reaction between the unknown amine and iodomethane was run and the precipitate was filtered and dried.
The melting point of the precipitate was taken and compared to the list of the literature values of the melting points for the corresponding salt. Based on that comparison, the unknown amine was identified to be benzyl trimethylammonium iodide. Conclusion In the experiment, the theory behind an SN2 reaction was used to run seven different reactions. The reactions were run to test how different factors affect the rate of the SN2 reaction. The factors that were tested are steric hindrance, nuclophilicity, and the nature of the leaving group. The seventh reaction was run to get the precipitate from the SN2 reaction of an unknown amine. The precipitate was then analyzed and the melting point of it was determined in order to identify the unknown amine.
All the reactions were successfully run and observed. The results of the reactions were discussed and the reason why Reaction 3 did not give any observable results was discussed. The rates of the reactions were compared and the differences analyzed. Based on these differences, the effects of various factors on the SN2 reactions were determined and discussed. The unknown amine was successfully identified. Overall, the experiment was successful and accomplished what it was set to do. In real life, such reactions are used to synthesize substances for the use in industry and science. Since a lot substances can be received a product of an SN2 reaction, the mechanism is widely used.
